Many types of agricultural or construction machines, such as tractors or telescopic loaders, have a hydraulic system which includes a hydraulic consumer, such as a hydraulic cylinder or hydraulic motor or other hydraulically driven components. Such hydraulic systems include hydraulic pumps that are connected with a drive shaft of the engine directly or over a rigid connecting gearbox that can be shifted into a fast or slow gear ratio. Thereby the maximum volume flow of the hydraulic pump varies with the rotational speed of the engine. The more rapidly the engine rotates, the larger is the pump flow rate. With adjustable or variable displacement hydraulic pumps, the maximum flow rate may be made to conform to the demand of the hydraulic consumer. This is usually performed by means of a flow rate controller which controls or maintains a predetermined control pressure difference between the pressure at the outlet of the pump and the load sense (LS) signal reported by the consumer. The flow rate controller of an LS controlled pump operates so that it controls the flow rate of the pump so that the predetermined control pressure difference that can be adjusted in a fixed manner by means of an adjusting spring and is maintained as a constant value at all times. The operation of such a pressure/flow rate controller is well known in the state of the art.
The flow rate that can be delivered by a hydraulic valve to a hydraulic cylinder or a hydraulic motor is a direct function of this control pressure difference. A certain control pressure is adjusted by means of the adjusting spring and an adjusting piston of a flow rate controller in that it forces the pump to maintain a control pressure difference corresponding to this adjusted pressure between the outlet of the pump and the LS signal. In order to attain this control pressure difference it pivots the flow rate control adjusting member to begin to convey a corresponding flow rate that can be controlled or adjusted as a function of the adjusting piston. The adjusting piston is connected hydraulically with the flow rate controller and changes its position as a function of the control pressure difference existing or provided as input at the flow rate controller. The flow rate control unit may for example include a pivoting disk that is connected with a control or lifting piston where the rotating movement of the pivoting disk is converted into a linear movement of the lifting piston. The flow rate conveyed by the pump flows through the lines and the valves of the hydraulic system and thereby generates certain pressure losses in the lines and in each of the valves leading to the consumer. The pressure that then develops behind the valves or at the consumer is transmitted back to the pump over a load pressure line that is connected with the flow rate controller, and induces the pump to convey as much volume flow as needed so that the pressure at the outlet of the pump is higher by the control pressure difference as the load pressure at the consumer delivered by the L-S signal.
The further a valve is distant from the pump, the larger will be the pressure losses due to the longer flow distance, which results in the effect that valves that are further removed from the pump as compared to other valves permit less volume flow to reach a consumer although these are valves of the same configuration. In order to compensate for this effect, known practice is to apply valves that report a re-enforced load signal to the pump, as is disclosed in EP 176 0 325 A2.
Accordingly, a certain pressure is required to force a certain volume flow through a line or a valve. Since the pressure losses increase with the volume flow, it would be advantageous to maintain the cross section of the lines and bores as large as possible and to keep the losses as small as possible in the configuration, if a certain amount of hydraulic fluid is to be made available to a consumer. If the losses now become too great and the volume flow is thereby reduced, this can be compensated for by enlarging the cross section at the valve openings, that is, volume flows can be changed; they can be increased or decreased by changes in the cross section at the valve openings.
Other possibilities of changing the volume flow aim at changing the adjusting force acting on the flow rate controller of the pump. In this way, EP 0 439 621 B1 discloses that for a precision operation of the hydraulic system, the control pressure difference at the pump can be reduced by manual operation of an adjusting force at the flow rate controller, this results in lower maximum volume flow in the hydraulic system or in the valves.
Now the problem is that it may be advantageous for environmental and economic reasons to operate a hydraulic system of an operating machine in the lower rotational speed range of the engine. This has the result that too little volume flow is then conveyed with today's pump sizes that are available for the application, which in turn leads to the application of larger pumps, so that at low engine rotational speeds large volume flows can be conveyed. As a result, at high engine rotational speeds, very large volume flows are conveyed (that cannot be reduced), and this leads to very large power losses in the overall power balance. These problems could be overcome, at least partially, by increasing the control pressure difference of the pump which finally would result in increased fuel consumption for the machine, since a certain power output is required or a certain volume flow is required in order to attain the control pressure difference. Moreover, there is the possibility of designing all lines and valves for the maximum pump conveying power, which would lead in turn to very high costs for the individual components and to space problems on the operating machine. EP 349 092 B1 discloses a further possibility to permit higher volume flows at low engine rotational speeds, but to limit the volume flow at high engine rotational speeds. Here the maximum volume flow conveyed by the pump is limited, so that the flow rate of the pump is measured and monitored, for example, by measuring the position of the flow rate adjusting mechanism, such as the adjustment angle of an adjusting disk or a pivoting disk. Such pumps and the corresponding electronic controls however are costly and expensive.